Graphite/carbon shapes such as electrodes for use in the conventional electric arc furnace and sliding contact devices such as brushes used in electric motors and the like are heated during use. The heating causes the carbon to react with oxygen in the atmosphere to form gaseous carbon monoxide and carbon dioxide, the reaction providing for deterioration of the graphite/carbon article beyond that caused by normal process consumption.
Over the last 20 years numerous researchers have addressed the problem of providing oxidation resistant coatings for graphite/carbon substrates. Particular emphasis has been placed on providing these coatings for graphite electrodes for use in the electric arc furnace. Prior attempts at solving the problem centered around depositing either silicon or silicon carbide coatings by a plasma spray or chemical vapor deposition process on the major cylindrical surface of the electrode. Other coatings involve using a silicon carbide slurry which is painted on and allowed to dry.
A paper titled "Recent Developments of Electric Arc Furnace Electrode Evaluation Protection at Armco, Inc.," by P. Shcroth, B. H. Baker presented at the First European Electric/Steel Congress Sep. 12-14, 1983, in Aachen, West Germany, summarizes the use of a coating (not defined) that is non conductive and can be sprayed or brushed on furnace electrodes. The coating resulted in savings of electrode material of between 10 and 20%. However, since the coating is non-conductive, it must be applied below the electrical clamp at the mill before the electrode is positioned for use in the arc furnace, thus requiring significant capital expense and changes in current operating practice for the electric arc furnace shop.
A paper by J. H. Courtenay and J. Helmut, titled "Lower Electrode Consumption Through Reduced Sidewall Oxidation", published in Fachberichte Huttenpraxts Metallweitervera bettung Volume 23, No. 10, 1985, provides a review of three techniques used to reduce sidewall oxidation of large electric arc furnace electrodes. The techniques include precoating, water cooling and in situ coating, the later coating technique called Platol. The coating system Is fully automatic with a robot spray applicator and is installed on the furnace and places a fusible glass matrix onto the electrode. The coating material consists primarily of silicon carbide and resulted in graphite savings of between 14 and 22 percent, with a net saving after costs reported to be approximately 10% of the graphite cost. The coating is non-conductive and requires a high capital investment in equipment.
An article by S. Dallaire appearing in "Surface and Coatings Technology", Volume 32, 1987, pages 141-142, discloses the use of a plasma coating technique to coat graphite electrodes. The powdered materials used to form the coating were aluminum, silicon carbide, titanium, and titanium carbide. The titanium-titanium carbide powders are sprayed first to a thickness of 50 micrometers to achieve a good bond. Then the aluminum-silicon carbide powders are sprayed to a thickness of 700 micrometers. The resulting coating was sensitive to temperatures above 800.degree. C. and thus did not remain Intact below the roof of the furnace. The main conclusion is that an adherent AL.sub.4 SiC.sub.4 layer in close contact with graphite provided the protection.
Other attempts have been made to coat graphite steel electrodes with very thick aluminum based materials by a plasma deposition process. The coatings 1 to 2 millimeters in thickness, reportedly decreased the wear of an electrode resulting from oxidation between 25 and 30% without negative effects on thermal or electrical properties.
A German published patent application 3609359 (20 Mar. 1986) discloses a coating process using plasma spray techniques in a vacuum chamber for electrodes. The process of the application uses silicon deposition to approximately 0.1 millimeters. The graphite electrode must be sandblasted using an inert gas and a controlled atmosphere must be employed during the lengthy cooling cycle to assure integrity of the coating.
Japanese published patent application (83/224281) discloses using an unidentified method to spray a powder mixture of SiC, Si.sub.3 N.sub.4 -phosphate --Cr.sub.2 O.sub.3 --TaC, AlAl.sub.2 O.sub.3 in a glass (ZrO.sub.2 -SiO.sub.2 --MgO--FeZO.sub.3) along with copper, nickel, stainless steel, iron, tin powder onto graphite electrodes. According to applicants, service life of the coated electrodes was increased by 11.7%. It is believed that a service life greater than 15% is necessary in order for any coating to be viable.
U.S. Pat. No. 1,098,794 discloses and claims deposition of titanium oxide or titanium oxide plus carbon and binders on a carbon substrate such as a furnace electrode, followed by baking at high temperature in the presence of nitrogen to convert the deposit into a titanium nitride coating. The post deposition operation in which the entire electrode must be uniformly heated in a nitrogen atmosphere prior to use, consumes time and energy and is capital and labor intensive.
U.S. Pat. No. 3,852,107 discloses and claims providing a thick coating comprising 15 and 90 wt % of a matrix material, having a melting point greater than 1,000.degree. and 10 to 85 wt % of a refractory filler which is electrically non-conductive and can be applied to the graphite electrode only below the electrode clamps. The coating must be reapplied every two hours or whenever the electrodes are moved down into the furnace, requiring work on hot furnace electrodes which are already installed in the furnace. Alternatively, the coatings can be prefabricated and applied as sheets which must be glued to the electrode.
U.S. Pat. Nos. 3,939,028; 4,301,387; 4,439,491; 4,567,103; 4,711,666: 4,772,514 and 4,487,804 disclose and claim various attempts at coating graphite/carbon electrodes for use in the electric arc furnace and are indicative of attempts to solve the troublesome problem of excessive electrode oxidation and/or mechanical wear at elevated temperatures.